1. Naming in Networking
Jennifer Rexford
COS 316 Guest Lecture

2. Names Type of Name Example Uniform Resource Locator
Hostname
Internet Protocol
Media Access Control
00:15:C5:49:04:A9
Today’s lecture focuses on the last three!

3. Internet Protocol Layers
Application
Applications
Hostname
Transport
Reliable streams
Messages
Best-effort global packet delivery
Network
IP address
Best-effort local packet delivery
Link
MAC address

4. Internet Protocol Stack
host
host
HTTP message
HTTP
HTTP
TCP segment
TCP
TCP
router
router
IP packet
IP packet
IP packet IP IP IP IP
Ethernet
interface
Ethernet
interface
Optical
interface
Optical
interface
Ethernet
interface
Ethernet
interface
Ethernet frame
Ethernet frame

5. What’s in a Name? Human readable? Fixed length? Large name space?
If end users interact with the names
Fixed length?
If names must be processed at high speed
Large name space?
If many nodes need unique names
Hierarchical names?
If the system is very large and/or federated
Self-certifying?
If preventing “spoofing” is important

6. Different Layers, Different Names
Host name (e.g.,
Mnemonic, variable-length, appreciated by humans
Hierarchical, based on organizations
IP address (e.g., )
Numerical 32-bit address appreciated by routers
Hierarchical, based on organizations and topology
MAC address (e.g., 00:15:C5:49:04:A9)
Numerical 48-bit address appreciated by adapters
Non-hierarchical, unrelated to network topology

7. Hierarchical Allocation Processes
Host name:
Domain: registrar for each top-level domain (e.g., .edu)
Host name: local administrator assigns to each host
IP addresses:
Prefixes: ICANN, regional Internet registries, and ISPs
Hosts: static configuration, or dynamic using DHCP
MAC addresses: 00:15:C5:49:04:A9
Blocks: assigned to equipment vendors by the IEEE
Adapters: assigned by the vendor from its block

8. Host Names vs. IP Addresses
Names are easier (for us!) to remember vs
IP addresses can change underneath
E.g., renumbering when changing providers
Name could map to multiple IP addresses
to multiple replicas of the Web site
Map to different addresses in different places
E.g., to reduce latency, or return different content
Multiple names for the same address
E.g., aliases like ee.mit.edu and cs.mit.edu

9. IP vs. MAC Addresses LANs designed for arbitrary network protocols
Not just for IP (e.g., IPX, Appletalk, X.25, …)
Different LANs may have different address schemes
A host may move to a new location
So, cannot simply assign a static IP address
Instead, must reconfigure the adapter
Must identify the adapter during bootstrap
Need to talk to the adapter to assign it an IP address

10. Hostname, IP, and MAC Hostname IP Address MAC Address Example
00:15:C5:49:04:A9
Size
Hierarchical, human readable, variable length
Hierarchical, machine readable, 32 bits (in IPv4)
Flat, machine readable, 48 bits
Read by
Humans, hosts
Internet routers
LAN switches
Allocation, top-level
Domain name assigned by registrar (e.g., for .edu)
Variable-length prefixes, assigned by ICANN, RIR, or ISP
Fixed-sized blocks, assigned by IEEE to vendors (e.g., Dell)
Allocation, low-level
Host name assigned by local administrator
Interface, by DHCP
or local administrator
Interface, by equipment vendor
whois –h whois.arin.net
Why both MAC and IP:
Need identifier to use in bootstrap process
IP address is topology-dependent, so cannot keep when host moves
- LAN was meant to handle many different network protocols (not just IP)

11. Directory: Translate Name to Address
link
session
address
path

12. Directory A key-value store Caching the response
Key: name, value: address(es)
Answer queries: given name, return address(es)
Caching the response
Reuse the response, for a period of time
Better performance and lower overhead
Allow entries to change
Updating the address(es) associated with a name
Invalidating or expiring cached responses

13. Directory Design: Three Extremes
Flood the query (e.g., ARP)
The named node responds with its own address
But, high overhead in large networks

14. Address Resolution Protocol (ARP)
Every host in a LAN maintains an ARP table
(IP address, MAC address) pair
Consult the table when sending a packet
Map destination IP address to dest MAC address
Transmit the IP packet within an Ethernet frame
Local Area
Network
00:15:C5:49:04:A9
78:9A:B5:23:5D:98

15. Address Resolution Protocol (ARP)
But, what if the key is not in the table?
Sender broadcast: “Who has IP address ?”
Receiver answer: “MAC address 78:9A:B5:23:5D:98”
Sender caches the result in its local ARP cache
Local Area
Network
00:15:C5:49:04:A9
78:9A:B5:23:5D:98

16. Address Resolution Protocol (ARP)
Managing the ARP cache
Storing all key-value pairs introduces overhead
Entries become stale (e.g., IP assigned to new host)
Remove an entry if not used for some period of time
Local Area
Network
00:15:C5:49:04:A9
78:9A:B5:23:5D:98

17. Directory Design: Three Extremes
Flood the query (e.g., ARP)
The named node responds with its address
But, high overhead in large networks
Push data to all nodes (e.g., /etc/hosts)
All nodes store a full copy of the directory
But, high overhead for many names and updates
Central directory server
All data and queries handled by one node
But, poor performance, scalability, and reliability

18. Distributed Directory Design
Hierarchical directory (e.g., DNS)
Follow the hierarchy of the name space
Distribute the directory, distribute the queries
Enable decentralized updates to the directory
Distributed Hash Table (e.g., P2P applications)
Directory as a hash table with flat names
Each directory node handles range of hash outputs
Use hash to direct query to the directory node

19. Domain Name System (DNS)
Properties of DNS
Hierarchical name space divided into zones
Distributed over a collection of DNS servers
Hierarchy of DNS servers
Root servers
Top-level domain (TLD) servers
Authoritative DNS servers
Performing the translations
Local DNS servers and client resolvers

20. Distributed Hierarchical Database
unnamed root
com
edu
org ac uk zw
arpa
generic domains
country domains
bar ac
in-
addr
west
east
cam 12
foo my
usr 34
my.east.bar.edu
usr.cam.ac.uk 56
/24

21. DNS Root Servers 13 root servers (see http://www.root-servers.org/)
Labeled A through M
A Verisign, Dulles, VA
C Cogent, Herndon, VA (also Los Angeles)
D U Maryland College Park, MD
G US DoD Vienna, VA
H ARL Aberdeen, MD
J Verisign, ( 11 locations)
K RIPE London (also Amsterdam, Frankfurt)
I Autonomica, Stockholm (plus 3 other locations)
E NASA Mt View, CA
F Internet Software C. Palo Alto, CA (and 17 other locations)
m WIDE Tokyo
B USC-ISI Marina del Rey, CA
L ICANN Los Angeles, CA

22. TLD and Authoritative DNS Servers
Global Top-level domain (gTLD) servers
Generic domains (e.g., .com, .org, .edu)
Country domains (e.g., .uk, .fr, .ca, .jp)
Managed professionally (e.g., Verisign for .com .net)
Authoritative DNS servers
Provide public records for hosts at an organization
For the organization’s servers (e.g., Web and mail)
Can be maintained locally or by a service provider 22

23. Using DNS Local DNS server (“default name server”) Client application
Usually near the end hosts who use it
Local hosts configured with local server (e.g., /etc/resolv.conf) or learn the server via DHCP
Client application
Extract server name (e.g., from the URL)
Do gethostbyname() or getaddrinfo() to get address
Server application
Extract client IP address from socket
Optional gethostbyaddr() to translate into name

24. authoritative DNS server
DNS Queries
Host at cis.poly.edu wants IP address for gaia.cs.umass.edu
root DNS server 2 3
TLD DNS server 4
local DNS server
dns.poly.edu 5 7 6 1 8
authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu
Recursive query: #1
Iterative queries: #2, 4, 6
gaia.cs.umass.edu

25. Recursive vs. Iterative Queries
root DNS server
Recursive query
Ask server to get answer for you
E.g., request 1 and response 8
Iterative query
Ask server who to ask next
E.g., all other request-response pairs 2 3
TLD DNS server 4
local DNS server
dns.poly.edu 5 7 6 1 8
authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu

26. DNS Caching DNS query latency Caching to reduce overhead and delay
root DNS server
DNS query latency
E.g., 1 sec latency before starting a download
Caching to reduce overhead and delay
Small # of top-level servers, that change rarely
Popular sites visited often
Where to cache?
Local DNS server
Browser 2 3
TLD DNS server 4
local DNS server
dns.poly.edu 5 7 6 1 8
authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu

27. DNS Cache Consistency Cache consistency DNS design considerations
Ensuring cached data is up to date
DNS design considerations
Cached data is “read only”
Explicit invalidation would be expensive
Avoiding stale information
Responses include a “time to live” (TTL) field
Delete the cached entry after TTL expires

28. Setting the Time To Live (TTL)
TTL trade-offs
Small TTL: fast response to change
Large TTL: higher cache hit rate
Following the hierarchy
Top of the hierarchy: days or weeks
Bottom of the hierarchy: seconds to hours
Tension in practice
Set low TTLs for load balancing and failover
Browsers cache for seconds

29. Negative Caching Broken domain names are slow to resolve
Misspellings like and
These can take a long time to fail the first time
Remember things that don’t work
Good to remember that they don’t work
… so the failure takes less time in the future
But don’t remember for too long
Use a time-to-live to expire

30. DNS Reliability DNS servers are replicated
Name service available if at least one replica is up
Queries can be load balanced between replicas
Retransmission of lost queries
No response to a query? Try again!
Try alternate servers on timeout
Exponential back-off when retrying same server

31. Conclusions Network names Network directories
To identify remote end-points
Readability? Format? Length? Hierarchy?
Hostnames, IP addresses, and MAC addresses
Network directories
Key-value stores to map name to address
Flooding (ARP), local copy, central server
Hierarchical (DNS) or non-hierarchical (DHT)
More on protocol layers in a few weeks!